Long Term Evolution (LTE) is a communication network technology currently under development by the 3rd Generation Partnership Project (3GPP). LTE requires a new radio access technique termed Evolved Universal Terrestrial Radio Access Network (E-UTRAN), which is designed to improve network capacity, reduce latency in the network, and consequently improve the end-user's experience. System Architecture Evolution (SAE) is the core network architecture for LTE communication networks.
LTE uses exclusively packet switched (PS) signalling. When a network operator wishes to introduce LTE, he will be unable to operate a complete LTE service from the first day. LTE will need to be rolled out gradually to replace existing technologies. In order to do this, LTE networks must have some way of interacting with networks that use other technology, such as circuit switched (CS) signalling. Single Radio Voice Call Continuity (SRVCC), described in 3GPP TS 23.237 and 3GPP TS 23.216, allows handover of a session from an LTE network to a CS network. A similar study (known as rSRVCC, or SRVCC return direction) is looking into transferring a call from CS to PS, and is described in TR 23.885v1.2.0.
While the description herein refers to a PS network as an LTE network, it will be appreciated that the description applies equally to other types of PS network.
Referring to FIG. 1, there is illustrated a scenario in which a User Equipment (UE) 1 is in a location 2 that has legacy CS coverage, and then moves to a location 3 that has PS coverage such as an LTE network 3.
Referring to FIG. 2, there is illustrated a signalling procedure that shows how such a handover may be performed. The following numbering corresponds to that for FIG. 2:    S1: The radio network 2 in which the UE 1 is located detects that a handover may be possible/required and notifies the Mobile Switching Centre (MSC) Server 4 that is enhanced for rSRVCC.    S2. The MSC server 4 initiates a preparation procedure for the handover, which usually involves reservation of resources in the target network to make sure that the transfer can complete successfully.    S3. Once this has been done, the MSC Server 4 notifies the UE 1 about that a handover to the PS network 3 is to take place.    S4. The handover takes place. In step S4a, the network starts to send media towards the UE 1 over the PS access network 3 instead of over the CS access network 2. In step S4b, the UE 1 tunes in to the PS target network 3 and tries to re-establish the communication session control over PS (by sending an INVITE towards the network 5). The media can now be sent and received over the PS access 3.
One of the assumptions and prerequisites for rSRVCC is that the UE 1 has an active IP Multimedia Subsystem (IMS) registration when the transfer from CS to PS is performed. In practice, it may be difficult for the UE 1 to maintain the registration during a call, for example if the UE 1 starts the call in a CS access network 2 where the PS access network 3 can not be used at the same time as the CS access network 2. This is a particular problem for GSM EDGE Radio Access Network (GERAN) where DTM (Dual Transfer Mode) is not supported. TR 23.885v1.2.0 (clause 6.3), proposes to handle this case by attempting to maintain the registration for the UE 1 over the PS access network 3. However, this is complex and may be costly.